Conventional methods of patterning into metal-based substrates involve complex multi-step processes including deposition of the nascent metal-compounds on suitable substrates and patterning into such compounds.
Techniques commonly employed to deposit metals or metal oxides are spin-on deposition of metal precursor (sol-gel), evaporation, sputtering, and chemical vapor deposition (CVD). Each of these techniques has several limitations and hence has limited commercial viability.
Sol-gel is one of the most common methods of depositing a metal oxide layer. The method employs spin-on coating of a sol precursor dissolved in a suitable solvent followed by heating the substrate to a high temperature to convert the precursor film into metal oxide. The method is not very practical in the sense that it employs quite high temperatures. High stresses related to cycles of heating and cooling involved in the process lead to defectivity. Moreover, additional processing steps are required to pattern small features into the material.
Deposition by evaporation involves heating of metal-compound to be deposited to high temperatures. Vapors of such materials are condensed on the substrate under vacuum using a screen or shadow to form fine patterns of the material. Deposition by evaporation has limited commercial potential due to high temperatures and high vacuum requirements.
Deposition by sputtering involves vaporization of the material to be deposited by bombarding with high-energy atomic radiation. Similar to Evaporation, the vapors of the materials can be deposited on the substrate by condensation. Utility of the process is limited due to high energy requirements and lack of precision in controlling film properties.
The process of deposition by CVD is even more expensive than sputtering or evaporation due to additional costs associated with the specialized equipment required for the chemical reactions prior to material deposition.
Formation of fine patterns into such metal-containing layers is achieved by additional multiple steps of imaging and etching of a photosensitive film deposited on such materials. First a photoresist or photosensitive film is applied on the metal-containing layer and dried at an appropriate temperature to remove a majority of casting solvent followed by image-wise exposure to actinic radiation to which such photoresist material is sensitive. In case of a positive acting photoresist, the exposed area of the film undergoes chemical reaction rendering it soluble in an alkaline developer. The action of developer leaves behind a fine pattern of the photoresist material. In the case of a negative acting photoresist, the exposed portion of the film undergo chemical reaction rendering it insoluble in a solvent suitable for removing unexposed area of the film, leaving behind a fine pattern of the photoresist film, which acts as an etch mask to transfer pattern into underlying metal-containing material.
Another method of forming fine patterns involves depositing a non-photosensitive metal-containing film, into a patterned substrate. The non-photosensitive metal-containing film is etched back to resolve the underlying pattern.
The techniques of metal-compound deposition as well as patterning are cumbersome and expensive. Ultra high purity materials are required for successful deposition. Moreover, the resolution of the pattern formed by some of the techniques is quite limited. Therefore, methods involving direct patterning into metal-containing layers are desired.
A photoresist-free, negative-tone method of direct patterning into metal-containing materials is described in U.S. Pat. Nos. 5,534,312; 6,071,676 and 6,972,256. The metal complex used in the method is photosensitive and undergoes low-temperature reaction in the presence of light of particular wavelength rendering the exposed portion insoluble in solvent/developer. Disadvantages of this method include limited choices of the starting metal-containing materials which demonstrate a sharp switch in solubility behavior upon exposure requiring very high exposure doses and often treatment of harsh solvents to remove unconverted material. Such harsh solvents also attack the exposed area, destroying pattern fidelity. This technique involves metal compounds that show significant absorbance at the exposure wavelength. High absorbance at the film surface leads to less light penetration into the bulk of the film resulting in non-uniform photochemical reaction; hence, chemical composition of the film is quite heterogeneous across the film.
More recently, Hill et al in U.S. Pat. No. 7,176,114, describe positive-tone pattern formation into metal-containing precursor layers. Materials used in the positive-tone method show sharper solubility contrast between the exposed and un-exposed areas of the film. However, most of the disadvantages noted with the negative tone-methods such as requirement of high energy dose and high exposure times make this method less pragmatic for real production environment.